Under conventional precipitation conditions silver halide emulsion grains rich in chloride are obtained showing a cubic morphology with (100) crystal faces. Special measures are required to modify this usual crystal habit and this is especially the case in the preparation of tabular grains rich in chloride containing (111) parallel major faces. Several publications reveal the use of so-called "growth modifiers" or "crystal habit modifiers". Klein and Moisar, in Berichte der Bunsengeselschaft 67 (4), p. 349-355, report inhibition of the growth rate of silver chloride by purine bases such as adenine. Claes et al., J. Photogr. Sci. Vol. 21 (1973), p. 39-50, showed that growth modifiers can be used to precipitate octahedral and rhombic dodecahedral silver chloride crystals and they attributed the crystal habit modification to variations in surface hydration caused by those additives; typical examples of these modifiers included adenine, thiourea, hypoxanthine, benzimidazole and benzothiazole derivatives. The mechanism of the growth modifying action of adenine was studied in detail by Szucs in J. Signal AM Vol. 6 (1978) No 5 p. 381-405. In Paper III-13 on the International Congress of Photographic Science Wyrsch reported a triple jet precipitation in the presence of ammonia and Cadmium ions. Depending on pAg and pH control, octahedral (111), rhombododecahedral (110) and cubic (100) crystal habits could be obtained.
In more recent patent applications tabular chloride rich emulsion grains are contemplated. So Wey U.S. Pat. No. 4,399,215 discloses tabular silver chloride emulsion grains with an aspect ratio of at least 8:1 and parallel (111) major crystal faces. Precipitation conditions include the use of ammonia. Rather thick tabular grains are obtained. Wey U.S. Pat. No. 4,414,306 discloses tabular silver chlorobromide grains with at least an annular region where the molar ratio of chloride to bromide ranges up to 2:3.
In U.S. Pat. No. 4,400,463 Maskasky describes the preparation of a new crystallographic form of tabular silver halide grains rich in chloride by performing the precipitation in the presence of a special peptizer having a thioether linkage and an aminoazaindene growth modifier. A preferred growth modifier is adenine. In this example the reaction vessel contained a rather high amount of chloride (0.5 molar) before the start of the precipitation and pH was adjusted to 3.0.
Maskasky U.S. Pat. No. 4,713,323 discloses the preparation of thin tabular grains (less than 0.35 micron) by a precipitation technique wherein at least a 0.5 molar concentration of chloride ion is present in the reaction vessel at the start and oxidized gelatin is used containing less than 30 micromoles of methionine per gram. In a preferred embodiment, illustrated by examples, a growth modifier e.g. an aminoazaindene like adenine is used. In this case the reaction vessel contained 0.5 molar chloride ions and pH was adjusted to 4.0.
Tufano U.S. Pat. No. 4,804,621 describes a process for preparing chloride rich tabular grains in the presence of aminoazapyridine growth modifiers represented by a general formula of which adenine and derivatives are excluded. A preferred compound is e.g. 4-aminopyrazoloE3,4,dlpyrimidine. In this example pCI was maintained at 0.7 and pH at 4.0. Houle and Tufano EP 0 430 196 discloses a method of stabilizing the crystal habit of this type of grains using outer bromide shells. However it can be expected that such a procedure will detoriate some of the specific advantages of chloride rich emulsions, e.g. the fast developability.
Unexamined Japanese Patent Publication (Kokai) 62-218959 describes a method for the preparation of chloride rich emulsion grains with average aspect ratio greater than 5:1 and (111) principal faces for at least 60% of the total grain projection area characterized by the presence of thiourea derivatives. The method is claimed to eliminate the stage of seed crystal preparation. In Unexamined Japanese Patent Publication (Kokai) 01-250943 a photographic material containing similarly prepared grains is claimed; the emulsion layer further contains a sensitizing dye.
In Takada U.S. Pat. No. 4,783,398 tabular grains rich in chloride are disclosed with an aspect ratio ranging from 2:1 to 10:1: they are preferably formed in the presence of a compound represented by a heterocyclic ring containing a sulphur atom.
The preparation of similar grains with an average aspect ratio of at least 5:1 in the presence of a gold compound, preferably combined with a Rhodium or Iridium compound, are described in the Unexamined Japanese Patent Publication (Kokai) 63-213836.
Unexamined Japanese Patent Publication (Kokai) 63-218938 combines tabular grains rich in chloride with the presence of a benzotriazole or mercaptotriazole derivative in the emulsion.
Unexamined Japanese Patent Publication (Kokai) 01-159646 claims the combination of tabular grains rich in chloride prepared in the presence of a crystal habit controlling agent, and a phenol type cyan colour coupler. The material is colour processed using a bleach-fixing bath.
Nishikawa U.S. Pat. No. 4,952,491 discloses a photographic material comprising a silver halide emulsion layer containing tabular grains rich in chloride showing an aspect ratio of at least 2:1, said tabular grains having been precipitated in the presence of a crystal habit controlling amount of a spectral sensitizing dye before and during nucleation and during precipitation.
In The Journal of Photographic Science, Vol. 36, p. 182, Endo and Okaji try to formulate an empirical rule for the modification of the crystal habit of silver chloride to form tabular grains. They conclude a.o. that especially 1,3-dimethyl urea and 1,3-diethyl urea are effective agents in order to obtain high aspect ratio tabular crystals with (111) faces.
In European Patent Application No. 90202792.9. filed 19.10.90, a method is disclosed for the preparation of silver halide tabular emulsion grains, containing at least 75% chloride, wherein at least 50% of the total projected area of all the grains is provided by said tabular grains, and wherein said tabular grains exhibit an average aspect ratio of at least 5:1, an average thickness not greater than 0.5 micron and an average diameter of at least 0.6 micron, by a process comprising the following steps:
preparing a dispersion medium containing a gelatino-peptizer, and a heterocyclic compound according to general formula (Ia) or (Ib), or one of their tautomeric forms, in a concentration ranging from 10.sup.-4 to 10.sup.-2 molar, said medium being adjusted to a pH between 5.0 and 9.0 and to a pCI between 1.0 and 2.0 by means of a chloride ions providing salt; formula (Ia) and (Ib) are: ##STR2## wherein Z represents the atoms necessary to form a fused on aromatic carbocyclic or heterocyclic, unsubstituted or substituted ring, e.g. substituted with alkyl, alkenyl, aryl, alkoxy, hydroxy, mercapto, carboxy, amino or halogen, and PA1 performing a silver halide precipitation comprising at least one double jet step by introducing in said dispersion medium at least one solution containing chloride ions and at least one solution containing silver ions in such a way that pCl is maintained between 1.0 and 2.0. pH is maintained between 5.0 an 9.0 and the concentration of compound (Ia) or (Ib) is maintained between 10.sup.-4 molar and 10.sup.-2 molar; PA1 removing excess of soluble salts by a wash technique performed at a pH value comprised between 4.0 and 9.0; this wash technique is preferably ultrafiltration. PA1 (1) preparing a dispersion medium containing a gelatino-peptizer, and performing the precipitation of the silver halide grains under the following conditions: PA1 (2) adding to the dispersion medium, during and/or after completion of the precipitation, an amount of a spectral sensitizing dye preferably ranging from 10.sup.-5 mole to 5.10.sup.-3 mole, and most preferably ranging from 3.10.sup.-5 mole to 2.10.sup.-3 mole per mole of silver halide at a pH lower than 4.0; in this way adsorbed crystal habit modifier is totally or almost totally desorbed from the tabular emulsion grains with maintenance of the tabular habit; PA1 (3) removing excessive inorganic salts by a procedure starting at a pH lower than 4.0. e.g. ultrafiltration, or flocculation by a polymeric agent or an inorganic salt followed by several washings and redispersion.
R is hydrogen or a substituent as defined for ring Z; n is 1 or 0, and PA2 Q represents carbon in which case n=1, or 0 represents nitrogen, in which latter case n=0. PA2 (a) the chloride ion concentration in the dispersion medium is lower than 0.15 molar at the start of the precipitation and remains lower than 0.15 molar during the whole precipitation, and pH is maintained between 5.0 and 9.0; PA2 (b) the precipitation is performed in the presence of a crystal habit modifyer according to general formula Ia or Ib: ##STR3## wherein Z represents the atoms necessary to form a fused on aromatic carbocyclic or heterocyclic, unsubstituted or substituted ring, e.g. substituted with alkyl, alkenyl, aryl, alkoxy, hydroxy, mercapto, carboxy, amino or halogen, and PA2 R is hydrogen or a substituent as defined for ring Z; n is 1 or 0, and PA2 Q represents carbon in which case n=1, or Q represents nitrogen, in which latter case n=0. PA2 This crystal habit modifier can be added at several different stages of the precipitation. It can be added as a whole in one portion or in several fractions. Preferably this crystal habit modifier is an adenine derivative and most preferably it is adenine itself.
In a preferred embodiment the heterocyclic compound according to general formula (Ia) or (Ib) is an adenine derivative, and in a still more preferred embodiment the heterocyclic compound is adenine itself.
However in the practical application of the method described in this European application serious problems arise concerning the developability of photographic materials containing tabular grains thus prepared. It was stated experimentally that a major part of the crystal habit modifier remains adsorbed at the surface of the tabular grains and that this phenomenon is the cause of the poor developability of those grains. As a result exposed and developed photographic materials containing such grains show inferior sensitometric properties such as an insufficient sensitivity, gradation and maximal density.
The present invention provides a remedy to the deficiencies of the application of the teachings of the European patent application explained above.
It is an object of the present invention to provide an improved method for the preparation of tabular grains rich in chloride.
It is a further object of the present invention to provide photographic materials containing tabular grains rich in chloride, which are readily developable and show excellent sensitometric characteristics.